Single photon emission computerized tomography (SPECT) and positron emission tomography (PET) are well known nuclear imaging systems in medicine. Generally, in nuclear imaging, a radioactive isotope is injected to, inhaled by or ingested by a patient. The isotope, provided as a radioactive-labeled pharmaceutical (radio-pharmaceutical) is chosen based on bio-kinetic properties that cause preferential uptake by different tissues. The gamma photons emitted by the radio-pharmaceutical are detected by radiation detectors outside the body, giving its spatial and uptake distribution within the body, with little trauma to the patient.
SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma-ray pairs that are emitted in coincidence, in opposite directions, due to electron-positron annihilations. In both cases, data from the emitted photons is used to produce spatial images of the “place of birth” of a detected photon and a measure of its energy. In PET, photon detectors also provide an indication of the time when a photon is detected.
An Anger gamma camera generally comprises a scintillation crystal, which when struck by a photon emits light, an array of photomultiplier tubes (PMTs) arranged in a conventional hexagonal matrix, for giving the x-y position of the detected photon, various processing circuitry, and a processing unit. Solid state gamma cameras generally include an array of pixel sized detectors and may be based on one of a number of different technologies. Solid state cameras suitable for use in SPECT and/or PET imaging are described in PCT Application No. PCT/IL98/00462 filed Sep. 24, 1998, now WO publication 00/17670 and PCT publication WO 98/23974, the disclosures of which are incorporated herein by reference.
Many SPECT and PET systems utilize one or two gamma cameras of either the Anger or solid state type to detect gamma rays.
SPECT and PET imaging couple conventional planar nuclear imaging techniques and tomographic reconstruction methods. Gamma cameras, arranged in a specific geometric configuration, are mounted on a gantry that rotates them around a patient, to acquire data from different angular views. Projection (or planar) data acquired from different views are reconstructed, using image reconstruction methods, to generate cross-sectional images of the internally distributed radio-pharmaceuticals. These images provide enhanced contrast and greater detail, when compared with planer images obtained with conventional nuclear imaging methods.
In general, it is desirous to have imaging systems that can be used both for PET and for SPECT, depending on the need.
A factor that influences the quality of imaging is non-uniform photon attenuation by intervening tissue, that is tissue through the which the gamma rays must pass before being detected by the gamma camera or cameras. Transmission scanning is a technique used to generate an attenuation map for correcting gamma images for non-uniform attenuation. In principle, gamma radiation from a known source, external to the tissue, is transmitted through the tissue to a corresponding scintillation detector. As in the cases of SPECT and PET, the external radiation source and the detector are rotated around the tissue, and transmission data from different angular views, coupled with tomographic reconstruction methods are used generate an attenuation map of the internal structure.
Two important points in generating an attenuation map are that transmission scanning must be performed for each patient individually, as patients differ in size and in internal structure and that transmission scanning should be performed on the same spatial registry as the PET or SPECT imaging, else it is difficult to correlate between the attenuation map and the PET or SPECT data.
Therefore, transmission scanning is generally performed simultaneously or concurrently with the PET or SPECT imaging, using the same detector system for the PET or SPECT imaging and for the transmission scanning. In many systems, use photons of different energies to differentiate between the transmission scanning photons and those of PET or SPECT imaging.
U.S. Pat. No. 5,900,636 to Nellemann, “Dual-Mode Gamma Camera system Utilizing Single-Photon Transmission Scanning for Attenuation Correction of PET Data,” whose disclosure is incorporated herein by reference, describes a system of simultaneous PET of SPECT imaging and transmission scanning. FIG. 1 illustrates a view in a transverse (x-y) plane, in which two transmission point sources 30A and 31A are on the same side as two gamma detectors 10 and 11 in the transaxial (x) direction. Point sources 30A and 31A are mounted outside the fields of views (FOVs.) of detectors 10 and 11. Such mounting avoids blocking the detectors and reduces transmission self-contamination (where radiation from a point source strikes the detector near it). A transmission scan across the entire axial width of detectors 10 and 11 is performed at each angular stop about the z axis. The aggregate effect of these transmission scans with the illustrated placement of point sources 30A and 31A is a transmission FOV (of each transverse slice) represented by a circle 70. The emission field of view (in each transverse slice) is represented by circle 72.
In this device, the allowable width (in the transverse plane) of the fan beam generated by point sources 30A and 31A is limited by detectors 10 and 11. Thus, the transmission FOV 70 is defined by two boundaries, an outside boundary and an inside boundary. The outside boundary is defined by the outer edges of the transmission fan beams 68 and 69 at each of the angular stops about the z axis, while the inside boundary is defined by the circumference of a circle 76, which represents a blind spot in the attenuation map. In order to prevent the gap from resulting in incomplete data acquisition, the computer system (not shown) causes the examination table (not shown) to move vertically and horizontally relative to the z axis in dependence on the angular positions of the detectors 10 and 11 about the z axis, in order to provide full coverage of the object of interest.